Flying over Kenya's Rift Valley at 19,000ft it's hard not to feel a pang of guilt. I am en route to Kibale, a fertile, mountainous region in western Uganda, but here, below the wings of the 737, the land is slowly dying. Starved of moisture, the Rift Valley's fertile savannah - like much of Africa these days - seems to be burning up, a victim of global warming and changing weather patterns.

I feel guilty because as a privileged westerner with an addiction to air travel I am at this very moment spewing carbon dioxide into the atmosphere at the rate of about 1kg a minute, emissions that, according to climate change experts, may be contributing to Kenya's faltering rains.

That is by no means the end of my sinning. Before boarding the 737 at Nairobi's Jomo Kenyatta airport, I flew long haul from London on a 747 - a nine-hour flight that added nearly two tonnes of CO2 to the atmosphere.

Three days before that I was on another 747 - this time returning from Cape Verde to Gatwick, a six-hour flight that came at the end of a two-week island hopping spree that saw me taking a total of eight planes and spewing a further five tonnes of CO2 into the atmosphere.

Leafing through Kenyan Airways's in-flight magazine I read that Africa's rainforests are vanishing faster than any others on earth. During the Eighties the continent lost 10 per cent of its green canopy. Since the Nineties, this process has accelerated, with 5m hectares of forest - an area equivalent to Togo - being destroyed every year.

The causes are many and varied: logging for hardwood, planting for sugar cane, illegal land seizures by farmers and cattle ranchers, or simply encroachment by ordinary Africans desperate for firewood for their stoves.

Whatever the cause, it is a vicious cycle. The more trees are cut down, the quicker soils erode and the faster streams and rivers dry up. The result is drought or, when the rains come, catastrophic floods, prompting refugees to flee to more fertile, food-secure areas where they pile further pressure on the land. But what if there were a way of interrupting this cycle, of guaranteeing not only the survival of Africa's remaining rainforests but of regenerating those areas which have already been stripped of timber?

Moreover, what if instead of the burden for this replanting falling on Africans the costs could be borne by corporations or international travel addicts - people like you and me - through a system of carbon offsetting? Wouldn't that be a win-win solution for everyone?

That is what I have come to Uganda to find out. Together with Frederic Courbet, a Belgian photographer based in Nairobi, I will be travelling with the Uganda Wildlife Authority (UWA) and representatives of Climate Care, an Oxford-based company that part-funds a carbon-offsetting scheme that is helping to reforest Kibale National Park. Set up in 1998 by green entrepreneur Mike Mason, Climate Care was one of the first companies to begin marketing offsets from the voluntary market to British consumers and corporate clients, such as the Co-operative Bank. In 2005, the Co-op paid Climate Care £250,000 to offset one-fifth of the carbon emissions arising from UK mortgage customers' homes - effectively 'neutralising', as the Co-op puts it, 50,000 tonnes of CO2. ('Our mortgages could not only save you money,' it boasts, 'they can help save the planet, too.')

Although technically a limited company, Climate Care does not have shareholders and, thanks to a complicated trust arrangement, claims to be able to avoid potential conflicts between its environmental and commercial aims. Sounds great. But is carbon offsetting really the answer to global warming? Is Climate Care really a company that is 'doing good' - as it claims on its website - or is it, as some environmentalists argue, actually making the situation worse?

To its critics, offsetting is no more than a clever accounting trick, a seductive and possibly counter-productive ruse to convince consumers they can get something for nothing. Far from deciding to insulate their lofts, invest in low-energy light bulbs and switch to bio-fuels, most consumers will simply continue their same old polluting ways, thinking they've done their bit for the environment. In other words, as Kevin Smith argues in his book The Carbon Myth, offsetting is capitalism's version of medieval 'indulgences' - a convenient way of assuaging the north's guilt while Rome, or in this case Africa, continues to burn.

Then there is the issue of how you calculate the carbon sequestered in a particular area of forest and put a value on the resulting 'credit'. Before I began researching this article I had no idea, as I am sure most British consumers don't, that when you click on a carbon calculator looking to offset the emissions of your recent shopping trip to New York, what you're actually buying is a guarantee that those trees will not only be planted but will survive for 100 years - the period required to absorb your emissions and thus render your flight 'carbon neutral'.

Not only that, but the voluntary market in carbon offsets is self-regulating, meaning it's potentially open to abuse. Little wonder then that radical ecologists dismiss carbon offsetting as 'Enron environmentalism' and label many of the companies who market offset and renewable-energy products as 'carbon spivs'.

As Jutta Kill of Fern, a group which campaigns for sustainable forest use and the rights of forest peoples, told me before I left for Uganda: 'Who can guarantee a tree planted in Uganda or Kenya will still be standing in 100 years? How many of the countries that host these projects have even existed for 100 years?'

I'm fond of trees - I once spent two weeks trekking through the Bolivian cloud forest in search of the Cinchona tree, a valuable anti-malarial. Shortly before leaving for Cape Verde I planted an olive tree in my garden in west London and was elated to find it flourishing on my return. Trees are good for the soul. More importantly, from the point of view of the voluntary offset industry, they capture people's imaginations.

Anti-offset campaigners recently challenged Climate Care's ethical credentials, claiming that workers in Kibale are being paid below subsistence wages and that together with Face (Forests Absorbing Carbon-dioxide Emissions), the Dutch foundation behind the Kibale project and another controversial reforestation scheme at Mount Elgon in eastern Uganda, they are ignoring the rights of local people who have traditionally accessed the forest for firewood, building materials and medicinal plants.

The criticism reached a crescendo in January when the BBC's Inside Out broadcast a report claiming the wages being paid to workers at Kibale were 'abysmal' and that viewers would do better to reduce their own carbon emissions than to buy offsets. Climate Care disputes these allegations and maintains their wages are fair.

Tom Morton, Climate Care's managing director, clearly finds such criticisms exasperating. He acknowledges that some people in Kibale may have complaints, but says that when he travelled there recently with the Co-op's head of ethics, the community was happy with the way the park was being managed and content with their access to its resources.

'If your aim is to pick holes in a project you'll always find a few people who are disgruntled, but my belief is Kibale is one of the best reforestation projects there is,' Morton says. 'The Ugandan government has taken the decision to restore a portion of its natural habitat and I'm happy to be part of that process.'

Morton certainly doesn't look like a spiv. Dressed casually in beige trousers and a light-blue shirt unbuttoned at the neck, he talks a green storm, telling me how he has installed solar panels at his home in Nairobi and taken other measures to reduce his carbon footprint.

We are sitting on the terrace of the Speke Hotel in downtown Kampala after an exhausting morning inspecting Climate Care's latest initiative - an urban stove project. In partnership with clean-air entrepreneurs, Climate Care is exploring a pilot energy-efficiency scheme to supply charcoal- and wood-burning 'rocket' stoves to householders in the greater Kampala area. The steel-clad stoves painted in the Ugandan national colours have a narrow ceramic fuel chamber linked to a wider cylindrical steel-clad pot container. Manufactured by the Urban Community Development Association (Ucodea) they can reduce a household's charcoal consumption by up to half.

Since 2000, Ucodea has sold 30,000 stoves in Kibuye and other Kampala neighbourhoods and is now scaling up production, with the aim of manufacturing 4,000 stoves a month. According to Ucodea's managing director, Kawere Muhammed, the stoves, which retail for 17,000 Ugandan shillings (£5) are extremely popular. In a country where most people still rely on the three-stone fire or wasteful braziers, the rocket design has been a revelation. 'It's amazing no one has thought to cook this way before,' Muhammed tells me. 'At first people are cautious, but when they see how much charcoal they can save they are very enthusiastic. In theory, we could supply the whole of Uganda.'

But for that to happen, Ucodea will need to find new sources of financing, hence Climate Care's involvement. According to Tom Morton, forestry-offset projects now account for less than 20 per cent of Climate Care's portfolio. 'When people say we're assuaging people's guilt in the north by planting trees in the south I just groan,' he says. 'As far as I'm concerned the debate has moved on.'

Morton points out that through the wasteful burning of wood and charcoal, the average Kampalan household consumes a tonne of charcoal a year and emits five tonnes of CO2 - the same as the typical British household. With the Kyoto process dragging its feet on the issue of wood and charcoal for cooking, Climate Care is hoping that stove projects will soon qualify for carbon credits under a new voluntary gold standard.

En route to Kibale the next day in a Toyota Landcruiser, courtesy of UWA, I cannot help but think Morton is tinkering at the edge of an insoluble problem. Weaving in and out of the Kampala traffic, matatus, their windows black with carbon, belch oily clouds of exhaust fumes into our path. Stalled momentarily in traffic, two children carrying small bundles of kindling approach and offer the firewood to the UWA driver. Even after we have left the city it is hard to escape the scent of charcoal or the ubiquitous stacks of firewood beside the road.

One of Fern's main objections to forestation projects is that the carbon credits simply increase the ecological debt of the north. The more fossil fuel a northern country uses, it argues, the more land it needs to offset its emissions. But in a country as hungry for firewood as Uganda, where the forests are already under intense pressure from encroachers, this is unfair and undermines global efforts towards sustainable development.

At Mount Elgon, where Face began planting trees in 1994, marketing the offsets to consumers via another Dutch company, Greenseat, this competition for scarce forest resources has sparked a bitter, on-going dispute. In 1993 the Ugandan government declared Mount Elgon a national park and forcibly evicted the indigenous people living within its boundaries. According to an investigation by a Kampala-based NGO, Climate and Development Initiatives (CDI), UWA rangers did not hesitate to use force. In 2002, UWA evicted a further 300 families. One indigenous group, the Benet people, is now suing UWA for the return of their land.

Jutta Kill claims that Kibale has a similar history of evictions, albeit (as at Mount Elgon) predating Face and Climate Care's involvement. She also claims that the salaries Face/UWA pay forestry workers are about half those of workers on nearby tea plantations. More- over, she said that when she visited Kibale and quizzed people, few seemed to understand the concept of carbon credits or that Face and Climate Care were businesses, not charities.

Located 320km west of Kampala, near the border with Congo, Kibale is part of the Albertine Rift region and an area of high species diversity and endemism. The 800km² park is home to 13 species of primate, including chimpanzees and the rare L'Hoesti monkey. It is also something of a haven for 'twitchers' boasting over 300 species of birds. More importantly, from the point of view of environmentalists, it contains 325 species of trees.

During Idi Amin's rule 15,000 hectares of Kibale's forest were destroyed by Bakiga settlers from southwest Uganda, apparently with the dictator's approval. But in 1992, when the park was formally gazetted, UWA evicted the pastoralists - some 30,000 in all - resettling them in a less fertile region in northern Uganda.

Depending on who you talk to, these evictions were either violent or largely peaceful. Whatever the truth, the years of encroachment certainly wrought havoc with the park's fragile ecology, encouraging the rampant growth of elephant grass and exposing the remaining hardwoods to forest fires.

It was against this background that in 1994, Face, which was originally set up to offset emissions from a coal-fired power station in the Netherlands, negotiated with the Ugandan authorities to replant 10,000 hectares inside Kibale as well as 25,000 hectares at Mount Elgon.

Face began by restoring Kibale's access road, establishing a tree nursery and building new offices and staff headquarters. Next, it hired local villagers to tame the elephant grass - a gruelling, labour-intensive job that entails repetitive cutting every month with hand-held pangas (machetes). Finally, after a year of this regime, it began planting the first pioneer species - bridelia, broad-leaved croton and Sapium ellipticum (the jumping seed tree) - followed a year or two later by intermediates like the East African cordia, Ugandan green wood and the African plum tree.

In 2000, having planted 3,000 hectares, Face handed over the day-to-day management to UWA, leaving it free to concentrate on selling carbon credits. One of its biggest customers is Climate Care. Between 2000 and 2006 the company acquired 274 hectares. This year, it is planning to plant a further 74 hectares, bringing its total holding in Kibale to 348 hectares.

Every five years, Face and Climate Care ask SGS-Qualifor, the world's leading verification and certification company, to inspect the forest and assess the carbon-sequestration levels. If SGS is happy with the quality of the plantings and the forest's management it issues Face/UWA with a Forest Stewardship Council (FSC) certificate. Alternatively, if it finds fault it can issue what are known as 'corrective action requests' (Cars). In theory, these Cars apply not only to the stewardship of the forest but to social issues, such as its impact on local communities and whether or not villagers are allowed access to the forest's resources. Similarly, depending on the quality of the plantings and tending, it can give the project a high carbon sequestration score or mark it down.

It takes us five hours to reach UWA's headquarters at Kibale. The final 50km is particularly gruelling, as at Fort Portal the tarmac runs out and we have to bump and grind along a dirt track. It is only when I spot a group of baboons sunning themselves on the verge that I realise we've entered the park proper. A few moments later, we swing into a drive beside a huge sign bearing UWA's logo - the Ugandan cob with a pair of crossed rifles - and the legend, 'Let the World Remain Green', which could be read either as a request or a fierce injunction.

UWA's headquarters are located on a hillside overlooking a glittering crater lake with stunning views across the park. Unlike Kenya's Rift valley, the land here is lush and verdant. Were it not for the avocado and papaya trees and adjacent tea plantations you could almost imagine yourself in Switzerland.

Charles Tumwesigye, UWA's chief warden, is a busy man. His responsibilities include not only Kibale but Queen Elizabeth National Park to the south, which the Queen is due to visit in November when she arrives in Kampala to open the Commonwealth heads of government meeting.

Distributing tea and biscuits, he introduces us to his forestry project manager, Richard Kigenyi, and his team of assistants. Although Tumwesigye has been in his post for just a year, he clearly takes the BBC's report personally, claiming it 'misrepresented' UWA's work. It is not true that local people have been denied access to the forest, he says; on the contrary, UWA has been working closely with parish councils and local community leaders to permit access to the park on a sustainable basis. In the past, he explains, 'encroachers' failed to respect the park's boundaries, sneaking into the forest to gather firewood and fell trees for timber. Now, if they wish to gather firewood, medicinal plants or building materials they must approach their parish council for a permit and pay a small fee - anywhere from 200 to 500UGS (60p-£1.50). That money is then used to fund community projects, such as schools, wells and the digging of elephant trenches.

We move on to the main point of contention: the claim that workers are underpaid. Tumwesigye says that depending on the time of year, UWA employs between 400-500 casual labourers to plant saplings, tend to existing plots and construct fire lanes, but that the budget is set by Face.

I press him further and he admits that pay has been an issue, and that he's recently asked Face if it could bring salaries more into line with those of tea workers.

Over the next day and a half we get a whistle-stop tour of the forest, inspecting the plantings and speaking to workers, villagers and community representatives. We travel in a convoy, with Kigenyi in the lead car with us, and a five-strong team of UWA forestry officials bringing up the rear. We are burning a lot of carbon.

The Ugandans are solicitous hosts, constantly checking that we are having a 'wonderful' time. But it is they who have determined the itinerary and whom we should speak to, and their presence makes it difficult to judge whether the opinions expressed by interviewees are independent or representative.

We are introduced to Silver Kashaija, whose village supplies workers for UWA/Face and who, as a member of Busiriba parish council, gets to decide how income from the park is disbursed (by law, UWA must set aside 20 per cent of gate receipts, currently pegged at $25, for the benefit of the local community).

Kashaija has nothing but good things to say about Face. They helped repair the access road to the park and brought much-needed jobs to the village he says. When I press him on the wage issue, he replies: 'The pay is good - many people can now afford bicycles.'

What about the project itself - what does he understand by the term 'carbon offsetting'? 'There are emissions in industrial countries,' he says. 'When we plant trees we balance the carbon that is being produced by these countries.'

He pauses, glancing at the UWA officials seated in his living room, before adding: 'For me Face is great.'

The following day, Kigenyi collects us early and we set off to inspect the carbon plots - or 'compartments' as he prefers to call them. There are 43 in all. Varying from three to 368 hectares they are located deep inside the park and stretch for 15km from north to south.

Kigenyi explains how encroachers planted fast-growing eucalyptus for firewood. The tree is not native and one of UWA's first tasks was to eradicate it, along with other 'exotics'.

Parking the car, we ascend a thickly forested trail. We are now in the oldest part of the forest, standing in the shade of an immense 700-year-old mahogany tree. The tree is an example of one of the 'climax' species that Kigenyi hopes will one day populate the whole park. He shows us a mark near the base of the trunk. 'In colonial times the British tried to cut it down but their chainsaw broke,' he informs us.

We climb to the top of a viewing platform. To the north and east there is no sign of humanity, just an undulating green canopy and, to the south, Lake George. Only when we turn to the west does the canopy give way to bare hillsides and, on the horizon, near the border with Congo, a belching cement factory.

The equatorial sun is already burning and we are eager to get on, to visit the Climate Care plot and taste guilt-free air. But first Kigenyi wants us to inspect the fire lanes and the damage caused by marauding wildlife. He shows us a tree that an elephant has felled to eat its leaves. Next, he leads us to a field where a team of eight men dressed in UWA blue overalls are cutting a swathe through the elephant grass. It is punishing work. I ask one of the men to put down his panga for a moment while I attempt to interview him. But I do not speak Roturo and with one of the UWA officials translating I get the same response as before - 'Great' and 'Yes, I'm happy with the pay.'

Finally, we reach the Climate Care plot. To my untutored eye it looks like any other part of the forest. There is no fence or other visible boundary, just a mixture of trees in various stages of development. We follow Kigenyi up a gentle incline, past a Combretum molle, a tree famed for its anti-bacterial properties. Its trunk is charred - the result of damage from an earlier bush fire - but somehow it has repaired itself with a new growth of coarse bark.

Soita James, one of Kigenyi's assistants, stoops down to show us where a new shoot has spontaneously seeded in the shade of its arbour.

'When we came here, this and another tree were the only two standing - the rest was elephant grass,' says James. 'Now, thanks to Face, one day this will all be forest.'

But where are the more recent trees, the ones UWA has planted for Climate Care, I ask? Kigenyi shows me a small tree in the combretum's shadow. It is a 10 metre-high Bridelia micrantha. Once again there is no marker, nothing to identify it.

I run my hands along its trunk while Frederic takes my picture. The bark is smooth and grey, the leaves glossy green. Could this tree and the others planted by Climate Care one day help to save the planet?

I ask Kigenyi when UWA planted it and he replies, 'Six years ago.' Just another 94 years to go then.

Kigenyi is clearly passionate about his work. He tells me that villagers are beginning to appreciate the forest and now report encroachers to UWA. 'Thanks to the forest, the streams are coming back and they have fresh water,' he explains. 'They understand that the forest is not only for them but for future generations.'

Kigenyi paints a bucolic picture and certainly during our short visit to Kibale and surrounding villages such as Nyabweya, Mabono and Bigoni we saw little evidence of obvious disgruntlement or need. But it was impossible for me to judge whether it was the whole picture, and while I was impressed with UWA's conservation work I couldn't help wondering whether, given time and the absence of Face, the forest would have regenerated anyway. And what of the 30,000 settlers who were evicted in 1992? Have they also become conservationists, or are they at this very moment plundering carbon stores in another part of Uganda?

Before returning to Kampala, we stop at UWA headquarters to say goodbye. Tumwesigye looks harassed and tired. The day before, we discover, he ordered UWA rangers to arrest encroachers who'd been grazing their cattle illegally inside Queen Elizabeth National Park. The encroachers are now in jail, but their arrest has become a major political issue, prompting the personal intervention of Uganda's president, Yoweri Museveni.

I press Tumwesigye on park conservation and the concept of carbon offsetting. 'Many Ugandans still do not understand it well,' he admits. 'We embraced the Face project because it is helping us replant trees in areas where they were cut down and because it brings benefits to people living around the forested area.'

But isn't he worried about the commitment to maintain the plots for 100 years?

'Our contract is for 99 years,' he corrects me. 'If we renege on it we can be sued. But in any case it is already our mandate.'

So UWA would have done it anyway? Tumwesigye hesitates, sensing a trap - Fern's argument is that African governments have a duty to protect gazetted forests and if they did the forests would regenerate naturally.

'I can say we are doing our best to ensure the forest will never be destroyed,' he says eventually, 'but of course in a country like Uganda we can never say that we will never have another president like Idi Amin.'

Later that afternoon at Entebbe airport, waiting to board the return flight to Nairobi from where I will take an overnight flight to Heathrow, I buy a copy of New Vision and read that a huge row is brewing over a forested reserve in Busoga, in eastern Uganda. Pastoralists have been illegally felling trees for firewood. As a result, a stream that serves a community on the edge of the reserve is drying up. According to the report, 70 per cent of the original 16,000-hectare reserve has already been deforested. I do a quick calculation: 11,200 hectares - more than the area UWA/Face are planting at Kibale.

To be fair, Climate Care acknowledges that 'planting trees' is not in itself a solution to climate change and that we also need to reduce our fossil fuel and greenhouse gas emissions. But it insists that, even in countries as inconsistent as Uganda, replanting deforested lands can help redress the imbalance.

Having flown more than 4,000 miles and wasted God knows how much petrol motoring through the rainforest I'm not so sure. I love trees, but the case for forest offsets still strikes me as insubstantial and, ultimately, as ungraspable as air.

How carbon offsetting works

The basic idea C02 released through fossil-fuel emissions can be 'neutralised' or 'offset' by planting trees that absorb the C02 gases, locking the carbon away in forests where it is safely sequestered, thus reducing global warming.

Sounds too good to be true... Groups like the WWF argue that carbon offsetting is based on a false premise and that carbon stored in trees or biological carbon is not equivalent to fossil carbon, as it will be released back into the atmosphere through fire, natural decay or harvesting.

I've heard of other kinds of offsetting... There are energy-efficiency projects, such as the stoves described in this article, as well as renewable-energy projects.

Forests regenerate, individuals put up wind farms - wouldn't some carbon-saving activities happen without intervention? Good point. Carbon-offset projects must demonstrate additionalilty, ie that no other schemes were already in place or that they wouldn't have happened anyway or as the result of natural processes.

If I offset, can I fly, drive and leave my telly on standby with a clear conscious? Sadly not. Tony Juniper, Friends of the Earth director, says that offsetting should be part of a 'hierarchy' of actions, at the top of which are lifestyle changes aimed at reducing domestic emissions (low-energy light bulbs, fewer car journeys, etc) and switching to bio fuels and so on.

So how much forest would we have to plant? Offsetting the UK's annual greenhouse-gas emissions would require planting an area of forest the size of Devon and Cornwall every single year - and maintaining those forests forever.